The present invention relates to scanning exposure apparatuses and device fabrication methods using the same, and more particularly to an exposure apparatus suited to expose a large mask.
Recent displays, such as those of personal computers (“PCs”) and televisions, have frequently used liquid crystal display (“LCD”) substrates. The LCD substrate forms a desired shape of patterned transparent thin-film electrodes on a glass plate using photolithography, and the photolithography uses a projection exposure apparatus for exposing an original pattern formed on a mask onto a photoresist layer on the glass plate via a projection optical system.
The projection optical apparatus includes those types of so-called step-and-repeat and mirror projection manners.
The projection exposure apparatus has been requested to expand its exposure area with recent demands for a larger LCD substrate. FIG. 13 is a schematic view of a principal part in a conventional scanning exposure apparatus of a common mirror-projection type. In FIG. 13, 1 denotes a mask, 2 a mask stage for scanning the mask 1, 51-53 a projection optical system, 3 a plate such as a glass plate, and 4 a plate stage for scanning the plate 3. UV reactive photoresist is applied onto a surface of the plate 3. Reference numeral 11 represents arc-shaped illumination light from an illumination system (not shown).
As illustrated, the illumination system 7 generates the arc-shaped illumination light 55 using an arc-shaped aperture or a slit aperture arranged just before the mask 1 or at a position that is optically conjugate with the mask 1. Alternatively, use of an optical element such as a cylindrical lens would also provide similar arc-shaped illumination light.
An XYZ coordinate system is not shown. The illuminated scanning exposure apparatus aligns a longitudinal direction of the arc-shaped illumination light 13 with an X-axis direction, a transverse direction or a scan direction of the mask stage 2, and the-plate stage 6 with an Y-axis direction, and a direction perpendicular to an XY plane with a Z-axis direction.
A brief description will now be given of an operational principle. Only portion 10 of a pattern on the mask 1 is projected and transferred, which is subject to the arc-shaped illumination light. The entire circuit pattern on the mask 1 is projected and transferred onto the plate 5 by scanning the mask 1 at a specified speed in a direction of an arrow 9, as well as scanning the plate stage 6 in a direction of an arrow 8 at a speed of the former speed multiplied by an imaging magnification of the projection optical system 4.
A stage control system synchronizes the mask 1 and the plate 5, and control their scans. After the entire circuit pattern is transferred, the plate stage 6 moves or steps by a specified amount in the directions X and Y to repeat a pattern transfer of many different locations on the plate 5 in the same way as described above, and to expose an area larger than the drawn area on the mask.
A scan exposure with a large mask effectively enables an exposure apparatus for a broad exposure area to expose a large plate, such as a large-sized liquid crystal panel, with high throughput, and requires a pattern on the mask to focus on the plate across the entire broad exposure area.
A negative plate, such as a mask, would deform due to its own weight when supported horizontally. According to the strength of materials, the gravity deformation of the mask is proportionate to the fourth power of the length of one side of the mask, and remarkably increases as the mask becomes larger. For example, the following equation provides the maximum deformation ymax of a level mask with both edges freely supported:ymax=(5wL4)/384EI  (1)where w is weight per unit length, L is a length of a supported interval, E is a Young's modulus, and I is a geometrical moment of inertia.
The maximum deformation of a level mask with both edges fixed is given by the following equation:ymax=(wL4)/384EI  (2)For example, a quartz mask with L=500 mm and a thickness of 10 mm has a maximum deformation ymax=30 μm with both edges supported, or ymax=6 μm with both edges fixed.
On the other hand, the mirror-projection type projection optical system has a depth of focus (“DOF”) of approximately ±30 μm.
Therefore, the gravity deformation of the mask cannot be negligible relative to the DOF of the mask pattern projected image, and would deteriorate transfer performance such as resolution.
Even when a mask support means 14 supports four surrounding sides of the mask on the same flat surface, as in FIG. 5, a mask plane complicatedly deforms, as shown in FIG. 6, providing different sectional shapes.